Link Adaptation (LA) is an important function in wireless communications systems. In LTE, the purpose of LA is to determine a modulation and coding scheme (MCS) that will maximize the transmitter's data rate under a given radio link condition. In a typical system that implements a retransmission scheme, e.g., a Hybrid Automatic Repeat Request (HARQ) scheme, the LA process is designed to select the highest MCS with which a target Block Error Rate (BLER) can be achieved after a number of transmissions. In the present disclosure, a transmission success or failure refers to the success or failure after a given number of HARQ transmissions. Typically, an MCS is selected for each transport block, which is transmitted every transmission time interval (TTI). The LA process selects an appropriate MCS based on the current radio link quality that is either measured by the transmitter or reported by the receiver in the format of channel state information (CSI).
For a LA process to work properly, the reported CSI should reflect the true channel condition. The CSI should also be consistently reported by the receivers, e.g., user equipments (UEs). However, the methodology of link quality evaluation has not been standardized and varies from vendor to vendor. Some UEs may report CSI that, after being interpreted by the eNB, represents a channel condition better than the true channel condition, i.e., optimistic CSI. Some UEs may report pessimistic CSI that represents a channel condition worse than the true channel condition. The eNB converts the received CSI into channel quality measurements, for example, Signal to Interference plus Noise Ratio (SINR) estimates. Inaccurate CSI reports lead to inaccurate SINR estimates.
An outer-loop control process has been introduced to correct SINR estimates. However, the outer-loop control process works only when the selected MCS is “normal.” A selected MCS is normal if the selected MCS is the highest MCS with which the target BLER can be achieved under the current channel condition. When the MCS is intentionally selected to be higher or lower than the normal MCS, the normal outer-loop control process would most likely fail to yield a converging or reliable SINR value.
The present application discloses methods and apparatus for improving the normal outer-loop control process to ensure that the outer-loop control process works properly no matter whether the selected MCS is normal, conservative, or aggressive.